DO NOT MERGE Update buttons state once UI is ready

* updateUi was not getting called for outgoing calls between
DIALING -> ACTIVE

* Adhere to UI directive that buttons representing disabled functionality
are hidden rather than disabled.

* Also fix issue where buttons are not updated for incoming call.

Bug: 20295789
Change-Id: I2606aa7b27c3dfb2de441737f1ecc2cc80ffdd7e